JN0-351 Practice Questions

Junos devices support various types of tunnels for different purposes 1 2 .

Option B is correct.  Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol network 1 .  Junos devices support GRE tunnels 1 .

Option D is correct.  IPsec (Internet Protocol Security) is a protocol suite for securing Internet Protocol (IP) communications by authenticating and encrypting each IP packet of a communication session 1 .  Junos devices support IPsec tunnels 1 .

Option A is incorrect. Spanning Tree Protocol (STP) is not a type of tunnel.  It’s a network protocol designed to prevent loops in a bridged Ethernet local area network 2 .

Option C is incorrect.  While Junos devices do support IP-IP (also known as IP tunneling), it’s not supported on all Junos devices 1 .

Option B is correct.  Flooding is a mechanism used in Layer 2 bridging where the switch sends incoming packets to all its ports except for the port where the packet originated 1 .  This is done when the switch doesn’t know the destination MAC address or when the packet is a broadcast or multicast 1 .

Option C is correct.  Learning is another mechanism used in Layer 2 bridging where the switch learns the source MAC addresses of incoming packets and associates them with the port on which they were received 2 3 .  This information is stored in a MAC address table, also known as a bridge table 2 3 .

Option A is incorrect.  Blocking is a state in Spanning Tree Protocol (STP) used to prevent loops in a network 2 .  It’s not a mechanism used in building and maintaining a Layer 2 bridge table 2 .

Option D is incorrect.  Listening is also a state in Spanning Tree Protocol (STP) where the switch listens for BPDUs to make sure no loops occur in the network before transitioning to the learning state 2 .  It’s not a mechanism used in building and maintaining a Layer 2 bridge table 2 .

Link-state update packets are used to flood link-state advertisements (LSAs) among OSPF routers. The destination IP address of these packets depends on the network type and the state of the interface. On broadcast and point-to-point networks, OSPF uses two IP multicast addresses: 224.0.0.5 for all OSPF routers and 224.0.0.6 for all designated routers (DRs) and backup designated routers (BDRs)12. If the interface state is DR or BDR, the link-state update packets are sent to 224.0.0.5. Otherwise, they are sent to 224.0.0.63. On non-broadcast networks, such as Frame Relay or ATM, link-state update packets are sent as unicasts to each adjacent neighbor3. Therefore, the correct answer is A and

D.  

B  is correct because the loops do not exist is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid.  A loop in BGP means that a route has been advertised by the same AS more than once, which can cause routing instability and inefficiency 1 .  To prevent loops, BGP uses the AS-path attribute, which lists the AS numbers that a route has traversed from the origin to the destination 2 .  The receiving router checks the AS-path attribute of the received route and discards it if it finds its own AS number in the list 2 . This way, BGP avoids accepting routes that contain loops.

C  is correct because the next hop is reachable is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid.  The next hop is the IP address of the next router that is used to forward packets to the destination network 3 .  The receiving router checks the next hop attribute of the received route and verifies that it has a valid route to reach it 3 .  If the next hop is not reachable, the received route is not usable and is rejected by the receiving router 3 . This way, BGP ensures that only feasible routes are accepted.

BGP attributes are properties that BGP uses for route advertisement, path selection, and loop prevention 1 .  There are four categories of BGP attributes 1 2 3 :

Well-known mandatory: Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers 1 2 3 .

Well-known discretionary: Supported by all BGP implementations, and are optionally included in BGP updates 1 .

Optional transitive: May not be supported by all implementations of BGP 1 .

Optional non-transitive: May not be supported by all implementations of BGP 1 .

The well-known mandatory attributes must be supported by all BGP implementations and must be included in every update 1 2 3 .  These include the AS path and next hop attributes 2 3 . Therefore, options A and C are correct.